The disposal of waste water sludge has become an increasingly difficult problem in recent years. With land fills becoming over filled, pressure from environmental groups mounting, and legislation directed at stopping ocean dumping, waste water from municipal sewage systems is often incinerated, thereby yielding inert ash material. By far, the overwhelming majority of such disposal is accomplished through the use of multiple hearth furnaces.
FIG. 1 shows a very high level conceptual block diagram of a conventional multiple hearth furnace 101 comprising eleven hearths 1 through 11. Hearths 1 through 11 are constructed to support the many pounds of sludge or other material to be incinerated. The sludge is fed in through an input port 119 and is thereby placed on the top of hearth 1. In some systems, the sludge may be fed through an opening to enter the second hearth instead of the top hearth, thereby allowing the top hearth to be used as an afterburner for emissions control. The remainder of the operation of multiple hearth furnace 101 serves to move the sludge to be incinerated through the hearths one through eleven until an inert ash to be disposed of exits the system through output port 114. The technique of causing the movement will be discussed later herein.
The eleven hearths shown in FIG. 1 are typically divided into three different major zones. These zones, from top to bottom, are termed the drying zone 120, the combustion zone 121 and the cooling zone 122. In the present example, the drying zone 120 comprises hearths 1 through 4 and is utilized to dry the sludge from a water content of approximately 70-85%, when the sludge is received through input port 119 in a typical waste water treatment plant, to a water content of approximately 45 to 65 percent by weight.
Once the sludge is dried enough to reach 45 to 65 percent liquid by weight, it is forced downwardly into the combustion zone 121 and combated. Most of the volatile material is combated in the upper hearths 5 and 6 of combustion zone 121, thereby producing temperatures in the range of approximately 1200 to 1900 degrees Fahrenheit. This removes most of the volatile portion of the combustible material and produces a material containing inert ashes and solid carbon residue. The lower hearths 7 and 8 are used to burn any remaining carbon. Thus, the combustion zone is sometimes considered two zones, an upper combustion zone for burning most of the volatile material in the sludge, and a lower combustion zone for incinerating the remaining carbon. In the present example, hearths 5 and 6 comprise the upper combustion zone, and hearths 7 and 8 comprise the lower combustion zone, thereby forming an entire combustion zone of four hearths.
After combustion, the sludge, now essentially all inert ash, reaches the lowest hearths 9 through 11 which make up the cooling zone 122, and exits from opening 114. The cooling zone includes air, sometimes forced in from outside of the system with a fan. The final product exiting from output port 114 is inert ash at a temperature of approximately 100.degree. F.
FIG. 2 shows a typical arrangement of four arms 201 through 204 on central shaft 115. Each arm contains a plurality of rabble teeth 210.
During operation, the central shaft 115 rotates and the arms 201-204 move around the hearth, with rabble teeth 210 forcing the sludge toward the center of the hearth where it may be forced through opening 206 to the next hearth below. As can be appreciated from FIG. 1, some of the hearths include an opening 206 of FIG. 2 in the center of the hearth, while others include the openings 116 at the outer edge of the hearth, as shown in FIG. 1. The rabble teeth 210 for each hearth are tilted inwardly or outwardly in such a manner that causes the sludge to be forced towards the outside of the hearth for those hearths where the opening is at the outer edge of the hearth, and towards the inside of hearth for those hearths where the opening is towards the inside of the hearth as in FIG. 2.
In conventional multiple hearth furnaces such as that depicted in FIGS. 1 and 2 hereof, the temperature required for each of the zones is, for the most part, manually controlled. Specifically, air is injected into the combustion zone, usually through the cooling zone, in a quantity which is sufficient to supply the required oxygen for proper combustion. Additionally, auxiliary burners may be provided on the furnace in order to make up any heat deficient in the drying or combustion of the materials.
In recent furnaces however, due to higher capacity and dryer feed materials, additional excess air is often pumped into the combustion zone. The excess air is required to offset the hotter burning, increased capacity furnaces, and specifically, in order to appropriately limit the peak temperature thereof. The introduction of additional air into the combustion zone brings with it several disadvantages.
One such disadvantage is that the additional air results in the consumption of additional energy to power the larger fans required to power the exhaust gas cleaning equipment. In addition, the higher oxygen concentration that results from air being pumped into the combustion zone causes an increase in the presence of nitrogen oxides in the exhaust gas, as well as the formation of melted residual ash near the end of the combustion zone. Moreover, the increased flow of air often results in extinguished combustion in the carbon burning zone which results in incomplete combustion. As a result, metal sulfides may be present in the ash exiting the multiple hearth furnace. Finally, the additional air being forced through the combustion chamber also leads to a quenching effect which causes lumps of partially dried but unburned material called sludge balls to pass through the incinerator and present themselves at the ash disposal system.
It is an object of the invention to provide a technique for increasing the efficiency of multiple hearth furnaces.
It is another object of the invention to provide for automatic control and adjustment of air flows in multiple hearth furnaces using flue gas recirculation.
It is an object of the invention to increase the efficiency of multiple hearth furnaces without introducing so much oxygen into the combustion zone such that nitrogen oxide emissions are increased significantly.
It is another object of the invention to reduce the melted ash (i.e.; slag) formed as the sludge makes its way through the numerous hearths.
It is another object of the invention to increase the capacity of a multiple hearth furnace.
It is still a further object of the invention to provide a technique for reducing or eliminating the formation of sludge balls present in the material as it presents itself at the lower most hearths.